1. Field of the Invention
The present invention relates to a tilt servo apparatus for use in a reproducing apparatus of an optical disc or the like which controls a tilt of an information recording surface of the optical disc relative to an optical axis of an optical pickup when reading recording information from the optical disc or the like.
2. Description of Related Art
Hitherto, in order to correct a tilt in the radial direction of an information recording surface caused by a curvature or a self weight of an optical disc, a reproducing apparatus of an optical disc such as a laser disc or the like is provided with a tilt servo apparatus. More particularly, as shown in FIG. 1, a tilt sensor which comprises a light emitting diode, photodetector elements provided on both sides of the light emitting diode along the disc's radial direction, and the like is provided for an optical pickup for reading information. A light emitted from the light emitting diode enters an information recording surface of the optical disc being rotating. Reflection light beams are received by the photodetector elements, respectively. A tilt error signal is formed on the basis of electric signals obtained by the photo detector elements. A tilt motor is properly driven in accordance with a magnitude of the tilt error signal, thereby inclining the whole optical pickup along the radial direction together with a guide rail to support the optical pickup so as to be movable in the radial direction, thereby performing a tilt correction.
On the other hand, a DVD (digital video disc or digital versatile disc) as an optical disc on which a large quantity of digital information can be recorded has been proposed in recent years in association with diversification of the kind of information and a demand for reproducing apparatuses which can accurately reproduce the recording information on the DVD is increasing. With the DVD, digital information such as motion pictures, computer information, or the like can be recorded on a disc having the same diameter of 12 cm as that of a CD (compact disc) at a recording density that is six to eight times as high as that of the CD.
As described above, since a considerably high density recording is performed on the DVD as compared with the CD or the Laser Disc, it is necessary to set a spot diameter of a laser beam to a value which is extremely smaller than that of the CD or laser disc in order to read pit information. The spot diameter of the laser beam is proportional to a wavelength .lambda. of a laser beam which is used and is inversely proportional to a numerical aperture NA of an objective lens.
Therefore, in the optical pickup which is used for the reproducing apparatus of the DVD, it is necessary to use a laser light source of a short wavelength .lambda. and an objective lens of a large numerical aperture NA as compared with those of a conventional optical pickup which is used for a conventional reproducing apparatus of the CD or laser disc.
When the wavelength .lambda. of the laser beam is shortened and the numerical aperture NA of the objective lens is increased for the DVD, however, a wavefront aberration (mainly, coma aberration) occurs even if the disc is slightly inclined and an angle of a disc surface deviated from the surface perpendicular to an optical axis of the optical pickup, what is called a margin for a tilting angle is reduced. When the recording information is read, therefore, it is necessary to correct the tilt more frequently as compared with the CD or laser disc.
As described above, with the reproducing apparatus of the DVD, there is a need for a tilt servo apparatus having a response time shorter than that of a conventional apparatus. Since the conventional tilt servo apparatus shown in FIG. 1 is based on the mechanical control by the driving of the tilt motor as mentioned above, the use of a large complicated mechanism is necessary, so that there are problems that the size of the apparatus as a whole as well as its cost increases, a response time against the control operation is limited (about 1 Hz), and the tilt servo may not always be operated certainly. There consequently is also a limit in reduction of a search time when searching the recording information by the optical pickup.